The present invention is in the field of cryogenic shipping containers.
Commercial cryogenic equipment manufacturing goes back more than five decades. Union Carbide Corp. was a pioneer in developing many of the design and manufacturing methods, many of which are still in use today. U.S. Pat. No. 4,154,363, filed in 1975 for xe2x80x9cCryogenic Storage Container and Manufacture,xe2x80x9d captures the essence of defining how such a vessel is made, and therefore its disclosure is specifically incorporated herein by reference. These kinds of containers were intended for the storage of liquefied gases like liquid nitrogen (LN2). They were constructed in sizes and materials meant to provide portability for the transport of liquid nitrogen or biological materials frozen in LN2.
A further improvement in storage containers, especially for safer transport of LN2 stored in the absorbed vapor phase, can be found in U.S. Pat. No. 4,481,779, filed in 1983 for xe2x80x9cCryogenic Storage Container.xe2x80x9d This patent introduced the design for a so-called xe2x80x9cdry shipperxe2x80x9d intended to transport frozen biological specimens with less risk of liquid nitrogen release.
Refinements continued with the issuance in 1994 of U.S. Pat. No. 5,321,955 for xe2x80x9cCryogenic Shipping System,xe2x80x9d comprised among other things of a dewar having a top opening with one or more specimen holders suspended within the dewar. Specifically, a specimen holder design with a mostly cylindrical, open-mouthed metal canister attached to a rod made partially of a non-metallic, low heat transfer material known as composite.
As recently as 1995, U.S. Pat. No. 5,419,143 issued for xe2x80x9cCryogenic Apparatus for Sample Protection in a Dewar.xe2x80x9d Principally, this patent provided a convenient and inexpensive conversion of cryogenic storage dewars for shipping, an improved ability to maintain samples in a cold state for longer periods of time and an improved sample holder with protection against a loss of liquid cryogen.
In all cases, as far back as these kinds of cryogenic storage and shipping containers go, the general concept for plugging the opening to the inner vessel was a loose-fitting, round vapor plug. This plug was made of closed-cell foam for insulation of the heat conduction pathway through the neck tube opening. The reason for making the foam plug slightly smaller than the neck tube, typically 0.1 inch or less in diameter, was to provide an escape path for boiling vapors and assure that no pressure build-up would occur inside the container holding cryogenic liquefied gas.
In each case the vapor plug and its plastic handle were purposefully kept from positively engaging the neck tube interior surface for fear of trapping boiling vapors leading to a pressure rise inside of the container. Thus, the plug and its handle would not create any tight fitting interference between itself and the neck tube.
In 2000, with the issuance of U.S. Pat. No. 6,119,465 for a Shipping Container for Storing Materials at Cryogenic Temperature, comprised among other things of a Dewar having a top opening with a removable and replaceable cap for enclosing the specimen holding chamber creating a vented seal, a first attempt was made at controlling the migration of boiling vapors within the container. While clever in its ability to provide a more secure means of holding the specimens within the interior chamber, the cap does little to aid in the thermal performance of the overall container design. A loose fitting foam spacer sits atop the specimen chamber beneath the cap to act as an insulator.
As use of cryogenic shipping containers grows, specifically the use of fully absorbed LN2 dry vapor shippers, the challenges of good thermal management through carefully controlled heat transfer become increasingly significant. Since almost all LN2 containers utilize double-walled vacuum vessels with high performance (super) insulation to minimize heat transfer through the vessel sidewalls, the top opening becomes a principle means of heat transfer. Perhaps half the heat leak comes from the top opening of the container, depending on its size in comparison to the overall vessel size.
Use of poor heat conducting materials such as closed-cell foam insulation for the plug has been the historical means of minimizing heat leak through the neck opening. It is fairly effective at reducing heat transfer by convection in the bulk open space by displacing the majority of the gaseous vapors. However, the perimeter space created by the purposeful gap between the vapor plug and the inside surface of the neck tube does allow a xe2x80x9cchannelxe2x80x9d of vapor migration to remain. This channel is designed to allow the boiling liquid vapors a path to escape the container without building hazardous internal pressure.
When cryogenic storage containers remain in their preferred upright (top end up) position, the typical vapor plug arrangement described previously works well. However, in transit during shipment it is often impossible to assure that the container will remain upright. Despite the creativity of some packaging design, it is almost inevitable that some number of cryogenic shipping containers will transit on their sides, or worse yet, upside down.
Accordingly, there is a long-felt need for an improved vapor plug for use in cryogenic shipping and storing containers that provides increased thermal performance, and especially for increased thermal performance when the cryogenic container is not in its preferred upright position.
By using unique, lightweight, low-cost, semi-disposable, cryogenically compatible polymer films in combination with the foam insulation materials for the plug, the vapor phase LN2 dry shipper according to the present invention overcomes the above-mentioned disadvantages of the prior art. This is accomplished in an inherently elegant, reliable, and inexpensive adaptation of the foam vapor plug, which will result in improved retention of absorbed LN2 vapors, enhance the shipper""s tolerance of non-upright orientation during transit, and increase reliability and safety, with fewer in-service incidents of loss of cryogen.
The present invention is generally directed to an improved thermal barrier for a Dewar vessel and a Dewar vessel containing the thermal barrier. The thermal barrier is an insulative vapor plug and a vapor barrier. The plug is sized so as to define an open space between it and the neck portion of the Dewar vessel to allow venting of vaporous cryogen from the inner vessel of the Dewar vessel through a Dewar opening. The vapor barrier provides an interference between the plug and the neck portion that disrupts venting of vaporous cryogen but does not form an airtight seal that would block venting.
In a first, separate group of aspects of the present invention, the vapor barrier is made up of multiple vapor barriers, preferably four or more, that provide multiple interferences that can create chambers between the plug and the neck portion. Each interference disrupts migration of vaporous cryogen as an incremental increase (e.g., 2 psig or less) in vapor pressure of each chamber causes the chamber to breach and then another incremental increase in vapor pressure of the liquid cryogen in the vaporous state is required to breach each successive chamber.
In other, separate aspects of the present invention, a vapor barrier is made of a cryogenically compatible material, such as a polymer film, that retains vaporous cryogen within the vessel despite its orientation. A surface protrusion can be provided for the plug to inhibit the mean free path of dense, boiling vapors through the Dewar opening. Multiple protrusions can be affixed to the plug (which can occupy a majority of the open space within the neck portion) by lamination so that they extend outwardly from an outer surface of the plug. A handle, which can be made of webbing material, can extend through the plug and be attached to the plug at a bottom point located beneath any laminations so that the plug can be removed from the vessel by an upward pulling force exerted on the bottom point. The handle can also be affixed to a canister assembly.
In still other, separate aspects of the present invention, an insulative vapor plug and a vapor barrier can be inserted into the neck portion of a conventional Dewar vessel to increase its holding time.
Accordingly, it is a primary object of the present invention to provide an improved thermal barrier for a Dewar vessel that can increase its holding time.
This and further objects and advantages will be apparent to those skilled in the art in connection with the drawings and the detailed description of the preferred embodiment set forth below.